one and once are pronounced differently from the related words alone, only and atone. Stressed vowels often become diphthongs over time (Latin bona → Italian buona and Spanish buena), and this happened in the late Middle Ages to the words one and once, first recorded ca 1400: the vowel underwent some changes, from ōn → ōōōn → wōn → wōōn → wŏŏn → wŭn.